Can DX8 cards run DX9?
No, it doesn't support DX9 features. You can run the game, but only with a DX8 render path. Halo is my best example of this. It has several render paths one can use. If I remember correctly, it has different paths for DX7, 8, and 9. You use a switch in the windows properties box to tell it which one to use.
Other games support lower cards by letting the GPU emulate the features. If your card doesn't support a shader or something else, then it will send the info to the CPU. I'm not sure if games still use this, but you used to see games that would say "1GHz" needed if you had a good video card, or a 2GHz CPU if not.
Please tell me how to get a DX7 card to support DX9, using DX9 features.